1. Field of the Invention
The present invention relates generally to a sensor for measuring a dielectric constant or a resistivity of a flowing liquid and, in particular, is useful for determining ethanol content of a fuel mixture consisting of hydrocarbons and ethanol.
2. Description of the Related Art
As a fuel for internal combustion engines, it is known to mix relatively large amounts of alcohols, such as methanol and/or ethanol, with hydrocarbons. To obtain the maximum efficiency with a minimum of pollution when using such alcohol-based fuels, the engine settings should vary as a function of the fuel composition. Accordingly, it is known to control the engine settings depending upon the fuel composition.
It is known that the dielectric constant of a hydrocarbon/alcohol fuel mixture depends upon the composition of the mixture, and that the dielectric constant varies in an approximately linearly relationship with the alcohol content of the mixture. Various apparatus have been proposed based upon this characteristic.
German published application 39 23 992 discloses a sensor including a capacitor formed of a solid cylindrical central electrode and a hollow cylindrical peripheral electrode which is located coaxially about the central electrode. The two electrodes are placed in a chamber through which flows a fuel mixture. The fuel flow is perpendicular to the axis of the electrodes and the outer electrode is provided with two diametrically opposed longitudinal slots which enable the fuel mixture to pass between the electrodes. The disclosed sensor is quite complicated and, if it desired to not introduce an undesirable pressure drop in the flow of the mixture, the sensor must be quite large.
German published application 38 43 177 discloses a sensor that includes a metal tube through which flows a fuel mixture. The metal tube forms an outer electrode of a capacitor and a solid cylinder held in an axial position by centering elements forms a central electrode of the capacitor. The tube which forms the axial electrode is closed by insulating plugs. The connection of the central electrode traverses one of the plugs and the mixture enters and leaves the tube via transverse passages. Not only is the disclosed sensor complicated, but it is also bulky if an excessive pressure drop in the fuel flow is to be avoided.